The present invention relates generally to high voltage current limiting electrical fuses and particularly to such fuses of the general purpose type which can effectively interrupt both low and high currents.
High voltage fuses generally have an insulating tubular casing closed at both ends by metal terminal caps. An insulating fuse core typically extends between the caps inside the casing. Wound helically on the fuse core is a main fusible element, which may be one wire or ribbon, or several in parallel, connected at the ends to the terminal caps. The space in the casing around the main element is filled with a tightly packed arc-quenching filler, such as quartz sand.
The main fusible element is typically a silver ribbon which is perforated at regular intervals to provide cross-sectional "necks" which will melt prior to melting of the ribbon proper when the fuse carries a high fault current. Thus a high fault current results in the generation of a number of regularly spaced arcs which interact with the filler to limit the fuse current in a controlled fashion as the main element is consumed by arcing, until eventually all current ceases. The cross-sectional necks in the ribbon are an important feature for preventing the establishment of only a single arc, or merely several which might consume the ribbon entirely to the terminal cap and damage the cap to result in a failure mode for the fuse. The necks sufficiently increase the ribbon resistance locally that each neck is certain to generate an arc at high fault currents.
Low fault current operation of such a fuse is a somewhat different matter. At low currents, the distribution of melting I.sup.2 t, the product of the square of the current and time, in the main element is not sufficiently determined by the necks to assure that arcs will be generated at all their locations. The thermal conductivity of the filler and the thermal gradients in the fuse as a whole now play a major role in determining which neck will be first to melt. If the first to melt is near a terminal cap, the main element may be consumed adjacent to the cap, and this may result in a failure. Therefore, it is common practice to place near the center of the ribbon length an overlay of a lower melting point solder. Prior to any melting of the ribbon, the solder melts and reacts with the ribbon chemically to increase its resistance, thereby assuring the initiation of arcing at that central point.
It is desirable for the clearing characteristics of the fuse to establish additional arcs after the initial melting of the central ribbon segment. For this purpose, it is known to provide two metal arcing clips mounted on the core, each to one side of the center of the fuse, and half the distance to the cap and spaced a predetermined critical gap from the main ribbon element. The clips are connected together electrically by an auxiliary wire element also wound about the core. As the initial central segment arc becomes elongated and thus has an increasing potential difference between its ends, the ribbon portions opposite the clips also have this potential difference, and thus a voltage half that value appears across each of the gaps. The gap voltage increases until the air breaks down and arcing results to melt the ribbon opposite the clips. The arcing is then maintained between the severed ribbon ends by the fuse current until the fuse clears. Control of the gap spacing, the number of clips, and the spacing of the clips along the length of the ribbon thus affords control of the arc generation at lower fault currents. The gap in present fuses is generally an air gap, the spacing being determined by accurate positioning of the ribbon relative to the clip by stops of some sort, or, for example, by interposing a porous woven glass fiber tape between the ribbon and clip and pressing the covered clip outwardly against the ribbon. In this way, the tape thickness determines and maintains the gap spacing, with the air in the tape interstices providing the dielectric for the gap.
One problem with a gap structure as described above is that it is difficult with an air gap to accurately and reliably control the breakdown voltage at the desired lowest possible voltage to assure that arcing is initiated as early as possible after the central segment of the main ribbon is severed. The tolerance of the gap must be very close. The inherent resilience of a woven tape, for instance, could lead to variations in the breakdown voltage of the gap spaced by it as a result of varying pressure against the tape by the ribbon element as it undergoes thermal cycling. The effects of undesired variations of the spacing upon the breakdown voltage are aggravated by the very high dielectric constant of air, which makes spacing highly critical.